1. Field of the Invention
The present invention is related to an optical component used for controlling a path of light, the optical component collecting light or diffusing light. The present invention is related mainly to a lighting device using this optical component and display device with the lighting device such as a liquid crystal television.
2. Description of the Related Art
In a recent large size liquid crystal television or a flat display panel, a direct-lit type lighting device and an edge light type lighting device are adopted. In a direct-lit type lighting device, a plurality of cold cathode fluorescent lamps or LEDs (Light Emitting Diodes) are regularly arranged in a rear surface of a panel as a light source. A diffusion plate having a strong light scattering property is used between an image display element such as a liquid crystal panel and a light source and thereby cold cathode fluorescent lamps or LEDs can not be viewed. On the other hand, in an edge light type lighting device, a plurality of cold cathode fluorescent lamps or LEDs are arranged at an end surface of a transparent plate which is called a light guiding plate. Generally, a light deflection surface is formed at a surface opposite to a light outgoing surface (which faces an image display element) of a light guiding plate, the light deflection surface efficiently guiding light entering from an end surface of the light guiding plate toward a light outgoing surface. Proposed light deflection surfaces are, for example, a component on which a white dot pattern is printed or a component on which a lens shape is provided, these having the purpose of efficiently guiding light toward a light outgoing surface.
Conventionally, in a direct-lit type and an edge light type lighting device, one or a plurality of optical films are arranged in order to improve brightness toward an observer of a display device.
A Brightness Enhancement Film (BEF, a registered mark of 3M (USA)) is widely used as a lens sheet as a means for improving brightness of a liquid crystal display screen. FIGS. 15-17 show a brightness enhancement film described in the following patent documents 1 and 2. In a liquid crystal display device 100 shown in FIG. 15, roughly, a light source 101, a BEF 102 as a bright enhancement film through which light emitted from a light source 101 passes and a liquid crystal panel 103 are arranged. As shown in FIG. 16, a BEF is an optical film in which unit prisms 105 having a cross sectional triangle shape are periodically arranged in one direction on a transparent base 104. This unit prism 105 has a larger size (pitch) compared with a wavelength of light.
A BEF 102 can collect light from “off-axis”, and redirect the light to the observer “on-axis” or “recycle” the light. That is, a BEF 102 can increase brightness on axis by reducing brightness off-axis when a liquid crystal display device 100 is used (viewed). Here, “on-axis” is a direction corresponding to a viewing direction F by an observer in FIG. 15, and is generally a side of a direction of a normal line to a display screen of a liquid crystal panel 103.
Further, in a display device described in a patent document 2, a light diffusion film is arranged between a lens sheet in which prisms are arranged in one direction and a liquid crystal panel, while a transparent convex shape dot is formed on a light ingoing surface of a lens sheet and thereby brightness can be improved and moiré interference between regularly arranged lenses and a liquid crystal pixel can be prevented.
A large number of display panels as shown in the following patent documents 1 and 2 are known as a display device in which a brightness controlling component is arranged between a light source and a liquid crystal panel, the brightness controlling component having a repetitive array structure of a prism represented by the above BEF 102. In the case where a BEF 102 is adopted, it becomes possible for a display designer to achieve desirable brightness on axis while reducing power consumption. In an optical sheet in which a BEF 102 is used as a brightness controlling component, light from a light source finally emits from a light outgoing surface at a controlled angle due to refractive function. Thereby it is possible to strengthen the intensity of light toward a viewing direction by an observer. On the other hand, recently, a micro lens sheet as shown in patent document 3 is frequently adopted. A micro lens sheet shown in patent document 3 is a sheet in which an approximately semispherical micro lens is irregularly arranged on a transparent base as shown in FIG. 18. In the micro lens sheet, problems such as a side lobe generated in a prism sheet and moiré interference as mentioned above do not occur. Its visual characteristics are similar to the visual characteristics of a surface diffusion sheet and its front brightness is higher than the front brightness of a surface diffusion sheet.                Patent document 1: JP-A-H10-506500        Patent document 2: JP-A-H06-102506        Patent document 3: JP-A-2006-301582        
However, in the case where a BEF 102 described in patent documents 1 and 2 is used, at the same time, a useless component of light due to reflection/refraction may be emitted toward a direction outside the viewing direction F′ of an observer. Regarding the light intensity distribution to the angle taken respect to the observer's viewing angle shown in FIG. 17, a broken line B shows a light intensity distribution of a BEF 105. In this case, light intensity in the case where an angle with respect to a viewing direction F′ is 0 degrees (which corresponds to a direction on axis) is highest. However, a small light intensity peak (a side lobe) is generated at an angle of 90 degrees with respect to a viewing direction F′. This side lobe is outside of a light path in a viewing direction and is uselessly emitted in a horizontal direction. Such an intensity distribution having a light intensity peak of a side lobe is not desirable and a smooth brightness distribution without a light intensity peak at ±90 degrees which is shown by a solid line A in FIG. 17 is desirable.
In addition, if brightness only on axis is excessively improved, a width of a peak of a curve of a brightness distribution becomes remarkably narrow and thereby a viewing region is extremely limited. Therefore, a light diffusion film which is different from a prism sheet has to be additionally used in order to appropriately broaden a width of a peak and remove moiré interference generated between periodical prisms and pixels. This increases the number of components, which is a problem. Further, since a prism sheet represented by a BEF 105 has a sharp top, a prism sheet is easily damaged in an assembling process or by friction with a light diffusion film, which has been previously been indicated as a problem. In addition, a micro lens sheet described in patent document 3 is less bright than a prism sheet and it was difficult for the micro lens sheet to be used for a backlight unit or a display device, which requires high brightness.